This invention relates to a video recording and/or playback system wherein the recorded video signal may be either a composite color video signal or a monochrome video signal and, more particularly, to apparatus used in such system wherein a mode control signal also is recorded to represent the type of recorded video signal so as to enhance a color killer operation during signal reproduction.
In a typical video signal recorder, such as a video tape recording system (VTR) intended for home entertainment use, a received color video signal is separated into its luminance signal component and its chrominance signal component, these respective components occupying different frequency bands. The luminance signal component is frequency modulated and the chrominance signal component, which includes the chrominance signals and a color burst signal, is frequency-converted to a lower band. Then, the frequency-modulated luminance signal component and the frequency-converted chrominance component are combined and recorded by, for example, one or more rotary magnetic heads. During reproduction, the recorded composite signal is separated into its respective components to enable the luminance component to be demodulated and the chrominance component to be frequency-reconverted to its original frequency band. Then, these restored components are combined and, typically, displayed as a color video picture.
In many of these video recording and/or playback systems, a color killer circuit is provided to suppress signals that might otherwise be transmitted through the chrominance channel during reproduction of a monochrome video signal. That is, when a color video signal is recorded and is to be displayed, the color killer circuit is disabled; but when a black-and-white video signal is recorded and is to be displayed, the color killer circuit is operated to suppress various harmonics and other signals that might be present in the chrominance channel. Typically, the determination as to whether a color or monochrome video signal is being recorded is dependent upon the level of the color burst signal. Since a color burst signal does not accompany a monochrome video signal, the absence of such a color burst signal may be assumed to represent that a monochrome video signal is being received. In many applications, a color burst signal detector is provided having a relatively low detecting, or threshold, level in order to provide a relatively sensitive color detector. It has been thought that this will enable accurate color signal recording and reproduction even if the color burst signal exhibits a low level. In this manner, the proper recording/playback mode, that is, a color or monochrome mode, will be established.
Unfortunately, the color detecting or threshold level may be set improperly with respect to a particular video signal that is being received. For example, if the threshold level is too low, noise that accompanies a monochrome video signal may be erroneously detected as a color burst signal. Hence, a color killer operation is defeated in this instance even though such an operation should be performed. Conversely, if the threshold level is too high, a low level color burst signal will not be detected, resulting in the erroneous performance of a color killer operation, whereby a color video picture will not be reproduced.
Operation of the color killer circuit is defeated erroneously when a monochrome signal having only a luminance signal component is recorded. This luminance signal component is recorded as a frequency-modulated signal, and its lower side band may fall within the frequency range normally associated with the chrominance channel. Thus, during reproduction, this lower side band may be interpreted as a frequency-converted color burst signal and, consequently, will be detected and used to defeat the operation of the color killer circuit. Thus, even though no chrominance signal component or color burst signal had been recorded, the color killer circuit nevertheless will be inhibited when the monochrome video signal is reproduced. This, of course, is contrary to the desired operation of the color killer circuit during black-and-white video picture reproduction.